The present invention relates to a zoom lens, and especially, to a zoom lens appropriate for video cameras, etc., which has a small F-number of 1.4, a high magnification ratio of 10 through 12, and is low cost.
A zoom lens which has groups of lenses having the order of positive-negative-positive-positive lens groups from the object side, and in which the second lens group has a variable magnification function, and the fourth lens group has a function of compensating for image plane movement caused by magnification change, has been disclosed by the present applicant (Japanese Patent Publication Open to Public Inspection No. 264902/1993). In this application, an example in which three elements of plastic lenses have been used as part of a 12 element lens has been disclosed. Further, the cost of the zoom lens has been further decreased as disclosed in Japanese Patent Publication Open to Public Inspection No. 180424/1994; and 5 elements of plastic lenses have been used as part of an 11 element lenses. Further decrease in cost of such types of zoom lenses is highly desired now.
On the other hand, it has been widely known that the number of lenses is decreased by using molded glass aspheric lenses. However, the cost of the aspheric lenses is currently still high, and actually, the cost of the zoom lens can not yet be decreased by the decrease of the number of lenses.
It could be assumed to use a larger number of plastic lenses as a means of further decreasing the cost of zoom lenses. However, generally, the refractive index of plastic lenses is lower than that of inorganic glass lenses, and kinds of plastic lenses are limited. Accordingly, it is difficult to compose a compact total lens system. Since it is difficult to conduct highly effective anti-reflection coating on plastic lenses, there are problems in which the loss of amount of light becomes large due to reflection from the lens surfaces, and the like.